1. Technical Field of the Invention
The present invention relates to a machine-top plasma discharge truing apparatus that trues a conductive grindstone with a special shape such as an extremely fine or thin shape on the machine, and fine-machining methods using the apparatus.
2. Prior Art
As optical telecommunications systems and optical technologies have rapidly progressed, hard brittle materials such as fine ceramics, optical glass, optical crystals, and semiconductor monocrystals have been widely used. Therefore, a technology for efficiently, accurately slicing or otherwise shaping these hard brittle materials is strongly demanded in the industrial field.
Electrolytic in-process dressing grinding (ELID grinding for short) is attracting attention as a processing method that is particularly suitable for forming such hard brittle materials. In the ELID grinding method, a conductive grindstone with extremely small or thin diamond grains, is used, and the workpiece is ground while electrolytically dressing the grindstone. The features include high machining accuracy, high-quality surface in roughness, and easy processing of three-dimensional hard parts.
Even with a grindstone shaped specially for microscopically fine machining work for extremely fine or thin shapes etc., eccentricity or deflection can occur during manufacturing. Therefore, before the grindstone is applied to such precision machining as the ELID grinding, eccentricity or deflection thereof must be removed by truing.
However, with the metal bond grindstone used in ELID grinding, the hardness of the bonding material is so high that the grindstone cannot be trued efficiently by conventional methods. In addition, correction accuracies are limited, so conventional truing methods cannot be used easily. More explicitly, when a grindstone applied to a hard brittle material either is extremely small, extremely thin (for instance, a diameter of 1 mm or less, a thickness of 1 mm or less) or has a complex shape, if a tool contacts the grindstone during mechanical truing, the grindstone body deforms, therefore, it cannot be trued to a high accuracy, which poses a problem.
On the other hand, electric discharge machining is known in the prior art as a non-contact machining method. According to this machining method, the workpiece to be machined is placed opposite a machining electrode in an insulative processing solution, with a gap, and the workpiece is machined to remove excessive portions by repeating short pulsive arc discharges.
In this machining method, however, there are problems including (1) the shape of an electrode must be preadjusted according to a desired processing shape, (2) spacing between the electrode and the workpiece should be controlled precisely, (3) large pulse current must be supplied between the electrode and the workpiece, which requires a large and complicated power supply, and (4) the electrode must be replaced frequently because its shape alters due to consumption of it.
The present invention has been achieved to solve the aforementioned various problems. That is, an object of the present invention is to provide a plasma discharge truing apparatus that can efficiently remove eccentricity and deflection of a grindstone with a special shape such as an extremely small or thin shape, does not deform, can true a workpiece to a high accuracy, needs only a small-sized, small-output power supply, does not require a complicated control circuit or device, and uses consumable parts that are easy to manufacture and remachine, such as electrodes, and to provide fine machining methods using the apparatus.
The inventors of the present invention noted that contactless, highly efficient, and accurate truing can be achieved by rotating a circular disk-like electrode while generating uniform, high-efficiency sparks (plasma discharge) between the outer rim of the electrode and the grindstone, and also that the power supply can be made compact with a small output capacity, and variations of electrode shape can be greatly suppressed. In other words, the conductivity of a metal bond grindstone used in ELID grinding is used to generate a plasma discharge at a microscopic gap between the grindstone and the electrode, thereby a metal bond portion can be dissolved and removed without contact at a high accuracy, therefore, the surface of the grindstone can be modified into a preferred shape. The present invention is based on the above-mentioned knowledge.
In more detail, the present invention provides a plasma discharge truing apparatus with a conductive grindstone (12) for machining workpiece (1), a circular disk-like discharge electrode (14) whose outer rim (14a) can access a surface (12a) to be machined by the aforementioned conductive grindstone, an electrode rotating device (16) that drives the above-mentioned discharge electrode to rotate around its shaft center Z, a position control device controlling a relative position between the outer rim of the electrode and the grindstone, a voltage applying device (20) for applying pulses of a predetermined voltage between the grindstone and the electrode, and a mist supplying device (22) for supplying pressurized conductive mist between the grindstone and the electrode.
According to the above-mentioned configuration of the present invention, the aforementioned sparks (plasma discharge) can be generated stably between the outer rim of the rotating circular disk-like discharge electrode (14) and the machining surface (12a) of the conductive grindstone (12) whose position is controlled by a position control device (18), thereby the metal bond portion of the conductive grindstone can be dissolved and removed highly efficiently and accurately, so the surface of the grindstone can be altered to a preferred shape.
Because the discharge electrode (14) rotates around shaft center Z by means of electrode rotating device (16), even if the electrode is consumed by a plasma discharge, the electrode can maintain a satisfactory roundness, even after it has worn by the plasma discharge, so that the electrode can be operated continuously for a long time.
In addition, a mist-supplying device (22) feeds pressurized conductive mist (more preferably, a mixture of slightly conductive aqueous solution and compressed air) between the grindstone and the electrode, therefore, compared to the case in a dry state or where an insulative liquid is directly supplied, the plasma discharge can be generated stably with a higher current at a lower voltage, and the power supply can be made more compact with a smaller output power. Furthermore, from an experiment, it was confirmed that when using the above-mentioned pressurized conductive mist, efficiency and accuracy of truing can be raised.
The present invention also provides a fine machining method with a plasma discharge truing process (A) wherein a circular disk-like discharge electrode (14) provided with an outer rim (14a) capable of accessing the surface (12a) to be machined by a conductive grindstone (12), and an electrode rotating device (16) that drives the aforementioned discharge electrode to rotate around a shaft center Z are provided, and while supplying a pressurized conductive mist between the grindstone and the electrode, DC voltage pulses are applied between the conductive grindstone and the discharge electrode, and the workpiece surface is shaped by the discharge; an electrolytic dressing process (B) wherein a dressing electrode (28) with an opposed surface (28a) separated from the machining surface of the above-mentioned conductive grindstone (12), and while supplying a conductive liquid between the grindstone and the dressing electrode, a DC voltage is applied between the conductive grindstone and the dressing electrode, and the conductive grindstone is dressed by electrolysis; and a grinding process (C) wherein the conductive grindstone machines the workpiece.
According to the methods of the present invention, a conductive grindstone with a special shape such as an extremely fine or thin shape, whose essentricity and deflection are removed by the plasma discharge truing process (A), is used to perform an electrolytic dressing process (B) and a grinding process (C) on the same machine either simultaneously or repeatedly, so that adverse effects of essentricity or deflection can be prevented, together with removing positioning errors that may occur during reinstallation of a workpiece etc., therefore, a hard brittle material can be machined highly efficiently and accurately.
Moreover, since pressurized conductive mist is supplied for discharge truing, as described above, the efficiency and the accuracy of truing can also be raised.
Other objects and advantages of the present invention are revealed through the following description referring to the attached drawings.